RO111740B1 - A catalyst preparation process which is used at the oxidative purification of the burned gases from the diesel engines - Google Patents

Abstract

The invention relates to a catalyst for the oxidative purification of exhaust gases of diesel engines with a high conversion efficiency for hydrocarbons and carbon monoxide and an inhibited oxidation effect with respect to nitrogen oxides and sulphur dioxide, containing a monolithic catalyst body which has ducts of ceramic or metal, through which free flow is possible, and is coated with an activity-promoting dispersion coating consisting of the finely particulate metal oxides aluminium oxide, titanium oxide, silicon oxide, zeolite or mixtures thereof as a support for the catalytically active components, the active components being present as platinum metals platinum, palladium, rhodium and/or iridium, which are doped with vanadium or are in contact with an oxidic vanadium compound. The reduced oxidation effect with respect to sulphur dioxide is achieved by the finely particulate metal oxides being surface-modified aluminium oxide, titanium oxide, silicon oxide, zeolite or mixtures thereof. The surface modification consists in the specific surface of the metal oxides being coated with a layer consisting of titanium dioxide or silicon dioxide and comprising from 1 to 5 monolayers.

Description

The invention relates to a process for obtaining a catalyst for oxidative purification of flue gases from Diesel engines, having a high conversion efficiency for hydrocarbons and carbon monoxide and reduced action on oxidation of nitrogen oxides and sulfur dioxide. The catalyst consists of a monolithic body with free channels that crosses the ceramic material or the metal that is covered with a dispersion layer that increases its activity, consisting of fine metal oxides, aluminum oxide, titanium oxide, silicon oxide, zeolite or mixtures thereof, as carrier substances for the catalytic active components, the active components being in the form of platinum, palladium, rhodium and / or iridium doped with vanadium or a vanadium oxide compound.

The flue gases from Diesel engines contain pollutant compounds that must be removed by proper purification of the flue gas. These compounds are carbon monoxide, aldehydes, hydrocarbons, polyaromatic hydrocarbons (PAK), sulfur dioxide, nitrogen oxides. The sulfur dioxide from the flue gas is formed from the sulfur content of approximately Q, 3% in the fuel for the diesel engines and leads to a share in the flue gas of 10-2OO ppm, depending on the load and the rotation of the engine.

In fact, Diesel engines result in a smaller amount of nitrogen oxides than Otto engines, but their share is still about 3 times higher than in the burned gas of an Otto engine after purification using a three-tiered catalyst. ways.

In addition to these harmful gaseous substances produced at ordinary flue gas temperatures of a Diesel engine of 225 to 350 ° C, Diesel engines also emit significant quantities of soot particles, depending on the mode of operation, consisting of a core. of soils and unburnt hydrocarbons, polyaromatic hydrocarbons (PAK), as well as metal compounds, water and sulphates absorbed on this core.

For the purification of flue gases from Diesel engines, the three-way catalysts used in Otto engines cannot be used, because flue gases from Diesel engines have a high oxygen content of 1 to 15% by volume. This results in air λ figures for flue gases from Diesel engines greater than 1. On the contrary, three-way catalysts need to oxidize hydrocarbons and carbon monoxide as well as for the simultaneous reduction of nitrogen oxides from air numbers. λ = 1.

In order to reduce the emission of flue gases from Diesel engines containing sulfur oxide and nitrogen oxide, filters based on ceramic pores with fine pores with alternately blocked channels (so-called leak filters on the walls) have been proposed, based on spongy ceramic substances, packages of wire braids, ceramic pipes, filters of ceramic fiber bales etc. With these filtration systems, the soot particles can be removed from the diesel engines from the flue gas stream. However, the particles separated by filtration from these flue gases can be burned only in a few operating situations, in which the temperature of the flue gas is high enough so that the filter can be regenerated.

In order to improve the regeneration behavior of the above filter systems, coatings with substances which reduce the ignition temperature of catalyst substances, such as vanadium pentoxide, vanadates, for example, AgV0 ₃ and perennials, are already known, these active substances being able to either endowed with a support material with fine granulation and possibly associated with a precious metal, such as platinum, to be introduced by impregnation (DEOS 3232729, DE-OS 3141713 and DE-OS 3407712.

meanwhile, it has been found that for wall leakage filters, which are widely used today, the conversion efficiency for

EN 111740 Hydrocarbons and carbon monoxide, especially for the low temperatures of flue gases from Diesel engines, even if these filters are coated with catalytic active components of the kind mentioned above. In addition, the use of leaking wall filters provided with a catalyst coated with carrier substances, has the disadvantage of high back pressure of the flue gas which adversely affects the efficiency of the engine, especially at high load with soot particles. Attempts to compensate for this disadvantage by higher catalyst loading have not been successful. The limited space conditions for most vehicles did not allow for an increase in geometric dimensions, which would allow the reduction of the back pressure of the flue gases.

in DE-OS patent no. 3940758 describes a catalyst that decisively improves the purification of flue gases from Diesel engines. This catalyst works continuously without particle separation and cyclical purification for oxidizing purification of flue gases from diesel engines with high conversion efficiency for hydrocarbons and carbon monoxide at low temperatures and reduced oxidation action with NO and SO ₂ . This catalyst contains vanadium compounds applied on small-grain aluminum oxide, titanium oxide and silicon oxide, zeolite as well as mixtures thereof, with fine granulation, as support substances and with platinum group metals as active components.

This catalyst, known in the art, has a high conversion efficiency for gaseous harmful substances at the same time as leakage of soot particles compared to the leak filters on the walls. The positive action of this catalyst is due to the uniform cell density, the catalyst having a honeycomb structure with long channels that the flue gas flows freely, reaching more often and effectively in contact with the surfaces of the catalytic coated channels, than in the case of leakage filters. walls, in which each component of the flue gas flows only once through the catalytic-coated porous wall, the flue gas being then pressed in the middle of the drainage channel through a given channel and through four neighboring channels through which the current filters penetrate.

The purified gases obtained with the oxidation catalyst of the flue gases from the described Diesel engines show a much greater improvement than those obtained with the filtration systems. However, taking into account the now more severe legal requirements for the flue gas purification of Diesel engines, a further reduction in particulate emissions is required, as well as an improvement in stability over time, the conversion remaining as high for harmful gases, ie for hydrocarbons, carbon monoxide and nitrogen oxides.

Since the agglomeration of soot particles depends to a large extent on the presence of sulphates in the flue gas, being intensified by their presence, measures must be taken to reduce the degree of sulfur oxidation from S0 ₂ to S0 ₃ of Diesel fuel.

The process of obtaining the catalyst used for the oxidative purification of flue gases from Diesel engines, according to the invention, eliminates the disadvantages of the known processes, in that it performs with stirring the metal oxides of aluminum, titanium, silicon, zeolite or their mixtures having a granulation. fine, in an alcoholic solution, with an organic titanium or silicon compound, after which the organic solvent is removed, the remaining solid is dried at high temperature, ground and calcined at room temperature.

300 ... 600 ° C, for 0.5 ... 4 h, when the titanium and / or silicon organic compounds are decomposed to the corresponding oxides.

The invention has a number of advantages, namely:

Provides an appropriate catalyst for the purification of the oxyRO 111740 Bl gas flue gas from Diesel engines with a high conversion efficiency for hydrocarbons and carbon monoxide and having a braking action of oxidation against nitrogen oxides and sulfur dioxide, and whose oxidation action of SO ₂ is reduced compared to the prior art. at the same time the catalyst exhibits improved long-term behavior.

The catalyst for the oxidizing purification of the flue gases of the diesel engines, according to the invention, has a high conversion efficiency for hydrocarbons and carbon monoxide and a braking action of oxidation against oxides of nitrogen and sulfur dioxide. The catalyst contains a monolithic body with freely flowing channels, of ceramic or metal material, which is covered with a dispersed layer that increases its activity from fine-grained metal oxides, aluminum oxide, titanium oxide, silicon oxide, zeolite or mixtures thereof as a carrier substance, the active components being present in the form of metals in the platinum, platinum, palladium, rhodium and / or iridium groups, doped with vanadium or in contact with vanadium compounds.

Metal oxides with fine granulation of aluminum, titanium oxide, silicon oxide, zeolite or mixtures thereof with modified surface, can be obtained by mixing in an alcoholic solution a preliminary step of titanium oxide and / or silicon oxide. The alcoholic solvent is removed by continuous mixing under reduced pressure and the remaining solid is dried at high temperature and after grinding for 0.5 to 4 hours it is calcined at 300 to 600 ° C, with the decomposition of the titanium oxide preliminary step and / or silicon oxide to titanium oxide, respectively silicon dioxide.

In this way, the properties of the surfaces of the metal oxides are advantageously modified. The metal oxides thus treated have a positive action on reducing the oxidation of S0 ₂ to S0 ₃ . Unlike the physical mixture described in DE6

OS 3940758, which is composed of aluminum oxide and titanium oxide (Degussa P 25, Rutil / Anatas mixture, specific surface area 51 m ² / g] titanium oxide applied, for surface modification, has a behavior in the rest of the metal oxides. long time much better.

For an efficient modification of the metal oxides it is necessary to provide their specific surface with a layer comprising 1 to 5 monolayers of titanium dioxide and / or silicon dioxide. The amount of TiO ₂ required to cover a specific surface of 100 m ² with a monolayer is calculated according to the Tan (Coated Silica as Support for Platinum Catalyst in Journal of Catalysis 129 (1991) 447-456) at 0.098 g Ti0 ₂ . As the titanium oxide and / or silicon oxide pretreatment, organo-titanium and organo-silicic compounds of the general formula Ti (0R) ₄ and Si (0R) ₄ , respectively R, having the meaning of an organic radical can be advantageously used. Suitable preliminary steps of titanium oxide are, for example, tetraethyl orthotitanate [Ti (0C ₂ H ₅ ) ₄ ], tetra-tert-butyl orthotitanate [Ti (0C (CH ₃ ) ₃ ) ₄ ], tetraisopropyl orthotitanate [ Ti (0CH (CH ₃ ) ₂ ) ₄ ] and tetra-propyl orthotitanate [Ti (0CH ₂ CH ₂ CH ₃ ) ₄ ].

Modified metal oxides should be applied to the body of the catalyst in a concentration of 30 to 250, preferably 75 to 180, especially 90 to 150 g / l volume of catalyst as a dispersion. Vanadium, calculated as V ₂ 0 ₅ , may be present in a concentration of 0.1 to 15 g / l volume of catalyst and metals in the platinum group in a concentration of 0.1 to 7 g / l volume catalyst. Monolithic, inert carrier substances, in the form of honeycombs with 5 to 100 cells / cm ² are suitable as catalyst bodies. Of the platinum group metals, platinum and / or palladium are particularly suitable.

The metals in the vanadium doped platinum group are obtained according to DE-OS 39

40758 by concomitant or successive impregnation in any desired succession of the dispersion coating which

RO 111740 Bl intensifies the activity with a solution of some metal compounds from the platinum metal group and with a solution of a vanadium compound, drying and possibly calcining at temperatures of at least 2DO ° C, preferably in a gas stream containing hydrogen . The impregnation with at least one of the two initial substances for the active component can take place before or after applying the activity intensification dispersion layer, on the inert carrier substance.

The catalysts according to the invention have a higher surface roughness than the comparison catalysts of the prior art, although in both cases metallic oxides with the same average particle size were used. The improved yield of catalyst conversion according to the invention can be explained in part by the higher surface roughness. Through the roughness of the surface the burned gas is more strongly swirled in the channels of the catalyst body and thus comes in more intense contact with the catalytic layer.

The following are examples of embodiments of the invention, in relation to the figures, which represent:

FIG. 1, graphical rendering of the aging cycle for the oxidation catalysts of flue gases from Diesel engines as a function of time;

FIG. 2a, measuring the surface roughness of the coating layer of a comparison catalyst VK1;

FIG. 2b, measuring the surface roughness of the coating layer of a catalyst K1, according to the invention.

In order to obtain the catalysts, according to the invention, cordierite catalyst bodies were used. They have a cell density of 62 cells / cm ² · with wall thicknesses of 0.17 mm.

Comparison example 1. A conventional catalyst of the state of the art conventionally noted VK1 was prepared as follows.

An aqueous coating dispersion with a solids content of 30% is prepared. The suspension contains, compared to the dry mass, 60% by weight aluminum oxide range (specific surface area 189 m ² / g) and 40% by weight titanium oxide (Degussa p 25; specific surface area 50 m ² / g; mixture of rutile / anatase). Then, a catalyst body is covered with metal oxides by immersion in the coating dispersion and after blowing the excess suspension to 120 ° C, it is dried in air. After a two hour calcination at 400 ° C, the coated body of the catalyst is impregnated with an aqueous solution of Pt (NH ₃ ) ₄ (0H) ₂ , dried in air at 150 ° C and calcined at 300 ° C. Thereafter, vanadyl oxalate impregnation, drying at 120 ° C and vanadyl decomposition at 500 ° C, take place in air. The preliminary stage of the catalyst thus obtained is reduced within 2 hours at 500 ° C, in the formation gas stream (95% N ₂ , 5% H ₂ ).

The finished catalyst contains 64 g of titanium oxide, 96 g of aluminum oxide, 5 g of vanadium pentaoxide and 1.77 g of platinum per liter of catalyst volume.

Comparison example 2. Prepare comparison catalyst VK2 analogously with comparison example 1, make a second comparison catalyst. instead of titanium oxide P25 from Degussa, anatas modified with specific surface area of 95 m ² / g is used.

Example 1. A catalyst according to the invention, denoted K, is obtained with the aluminum oxide gamma modified with Ti0 ₂

For the catalyst according to the invention, the same gamma aluminum oxide is used as in comparison example 1. For the modification of its specific surface with titanium oxide, the gamma aluminum oxide is mixed in an alcoholic solution (ethanol) with tetraethyl orthotitanate (( Ζ ^ Η ^ -φ / Π) After stirring for 2 hours the ethanol is removed by a rotary evaporator (water jet vacuum, T = 50 ° C) and the formed material is air dried at 120 ° C for 16 h. After grinding, the Ti0 ₂ / AI ₂ 0 ₃ powder formed, calcined at 400 ° C, for 4 h.

The powder of Ti0 ₂ / AI ₂ 0 ₃ thus obtained contains, in relation to its total weight, 60% by weight of aluminum and

RO 111740 Bl

40% by weight titanium oxide. This amount of titanium oxide corresponds to a specific surface coating layer of aluminum oxide with approximately 3 titanium dioxide monolayers, calculated according to Tan et al. (Coated Silica as support for Platinum Catalyst in Journal of Catalysis 129, (1991) , 447-456).

From the preliminary obtained Ti02 / AI ₂ 0 ₃ powder, an aqueous coating dispersion with a solids content of 30% is prepared. With this dispersion a catalyst body is covered, which is further processed as in Example 1. The finished catalyst contains 160 g Ti0 ₂ / AI ₂ 0 ₃ , 5 g V ₂ 0 ₅ and 1.77 g Pt, per liter of volume catalyst.

Example 2. A catalyst according to the invention, denoted K ₂ , is prepared, containing silicon dioxide modified with Ti0 ₂ .

The catalyst K ₂ is obtained by working as in Example 1. Instead of aluminum oxide silicon dioxide with a specific surface area equal to 260 m ² / g is used. The finished catalyst contains per liter of catalyst volume 160 g TiC ₂ / SiO ₂ composed of 40% by weight TiO ₂ and 60% by weight Si0 ₂ , each, compared to the total quantity of Ti0 ₂ / Si0 ₂ . The amount of Ti0 ₂ chosen is distributed in 2 monolayers on the specific surface of the silicon dioxide.

Example 3. Prepare a K _3- labeled catalyst containing SiO _2- modified aluminum oxide.

The catalyst according to the invention, K ₃ is obtained by working as in example 1. instead of tetraethyl orthotitanate (C ₀ H _{2 O} O ₄ T) tetraethoxysilane (CgHgo ^ Si) is used. The finished catalyst contains per liter of catalyst volume 160 g Si0 ₂ / AI ₂ 0g composed of 40% by weight Si0 ₂ and 60% by weight AI ₂ 0 ₃ , each, compared to the total weight of Si0 ₂ / AI ₂ 0 ₃ . The amount of SiO ₂ is distributed in 2 monolayers on the specific surface of the aluminum oxide.

Example 4. A catalyst conventionally denoted with K ₄ is prepared, working as in Example 3.

instead of aluminum oxide, 10 titanium dioxide with a specific surface area of 95 m ² / g is used. The finished catalyst contains per liter of catalyst volume 20% by weight SiO ₂ and 80% by weight TiO ₂ , each, compared to the total weight of Si0 ₂ / Ti0 ₂ . The amount of Si0 ₂ chosen is sufficient to obtain 2 monolayers of silicon dioxide on titanium dioxide.

Example 5. Diesel oxidation catalysts, according to comparison examples 1 and 2, as well as examples 1 4 were tested on the test bench of a stationary engine. The engine was equipped with a 4-cylinder Diesel engine (55 KW; 1.6 I lifting space) and a hydraulic test brake (Schenk AG type 230). Diesel fuel was used as a test fuel. traded with 0.2% sulfur.

At the starting tests, the conversion of carbon monoxide, hydrocarbons, nitrogen oxides and sulfur dioxide was measured, depending on the temperature of the flue gas before the catalyst at a spatial speed of 120000 h ' ¹ .

After testing, the fresh catalysts were allowed to age on the engine for 50 hours. The aging cycle used is shown in FIG. 1. One cycle lasted 60 minutes and was repeated fifty times for catalyst aging.

The results of the starting tests are shown in Tables 1 and 2. The results show a decrease of the conversion S0 ₂ at high temperatures for the catalysts Kt to K ₄ compared to the catalysts of comparison at similar conversion rates for CO, HC and NO _X.

Example 6. The catalysts described in Comparative Examples 1 and 2, as well as Examples 1 through 4, were analyzed using X-ray diffractometry, optical microscopy and surface roughness measurement. X-ray diffractometry studies were carried out on oxide powders of Ti0 ₂ / AI ₂ 0 ₃ , Ti0 ₂ / Si0 ₂ respectively Si0 ₂ / Ti0 ₂ . Oxide samples of the VK- catalyst! they showed in the fresh state the presence of the changes anatas

EN 111740 Bl and rutile in report 78/22. After aging for 7 hours at 65 ° C, the rate of part of the preferred catalytic modification anatas to 38% was reduced in air. A similar behavior had an oxide sample 5 of the comparison catalyst VK ₂ . Fresh ducklings 100% turned after aging at 650 ° C, to 65% in rutile. In contrast, in the oxide samples, according to the invention, it was found, both freshly and after aging for 7 hours at 650 ° C, in the presence of a 100% change of anatase (Table 3).

Optical microscope researches 15 of the covered walls of the channels showed to the comparison catalysts νκ _η and VK ₂ smooth and compact coating surfaces. The catalysts according to the invention K ₁ to K ₄ have rough and porous coating surfaces. This fact has been confirmed quantitatively by measuring the roughness with the Perthometer 58P roughness measuring device of the Mahr-Perthen company. Fig. 2a and 2b show explorations of the coating surfaces over a length of 0.8 mm for the comparison catalyst νκ _η (fig. 2a) and for the catalyst according to the invention, K ₅ (fig. 2b). The comparison catalyst exhibited an average square roughness R ₄ of only 0.58 pm, while it was at the catalyst, according to the invention, of 4.55 pm, more than 9 times.

Through the rough surface of the catalyst coating layer, according to the invention, Kt to K ₄ is formed in the local turbulent flue gas stream, through which the transport of substance and heat from the gas phase to the surface layer is accelerated.

Table 1

Starting test results on Diesel engine (55 kW, 1.61 lifting space, stationary test conditions) for catalysts Fresh (space speed = 120,000 h ' ¹ )

Catalyst T / 50% Convert 3590 ° C 450 ° C CO HC CO HC S0 _? NO _X SO p VK _n 220 221 91 82 13 2 21 VK ₂ 221 222 90 80 14 1 25 K 195 217 95 78 0 3 5 K ₂ two hundred 217 90 79 4 3 9 K ₃ 205 215 91 79 0 1 2 K ₄ 203 210 89 85 0 3 1

RO 111740 Bl

Table 2 Diesel engine start test results [55 kW, 1.61 lifting space, stationary test conditions] for catalysts after aging [spatial speed = '' 120,000 h ' ¹ )

Catalyst T / 5O% Convert 3590 ° C 450 ° C CO HC CO HC so _P N0 _K S0 ₂ VK, 236 242 91 77 5 2 11 VK ₂ 230 241 90 79 8 2 13 Kt 220 227 91 75 0 3 2 K ₂ 225 217 90 73 2 3 1 "3 218 218 91 70 0 3 2 K ₄ 221 220 90 71 0 2 1

Table 3

Results of X-ray diffractometric investigations for samples of the oxide coating dispersions (TiO ^ AI ^ Jg, Ti0 ₂ / Si0 ₂ and Si0 ₃ / Ti0 ₂ ) of the catalysts VK _1t VK ₂ , Κι, K _s , K ₃ , K ₄ .

Catalyst Fresh Anatas: rutile 7 h, 65 ° C, air, oven Anatas: rutile VK _n 78:22 38: 62 VK ₂ 100: 0 35: 65 K 100: 0 100: 0 K ₂ 100: 0 100: 0 K ₃ - - K ₄ 100: 0 100: 0

claims

Claims (5)

claims 1. Process for obtaining 40 of a catalyst used for the oxidative purification of flue gases from Diesel engines, formed by the catalyst body in the form of a free-channel crossed monolith, consisting of a ceramic or metal material, covered with a layer of dispersion of fine-grained metal oxides, such as aluminum oxide, titanium oxide, silicon oxide, zeolite or mixtures thereof, forming the support on which the catalytic active components are deposited, in the form of platinum group metals, palladium , rhodium and / or iridium, doped with vanadium or in contact with vanadium compounds, characterized in that it mixes, with stirring, the metal oxides of aluminum, titanium, silicon, zeolite or their mixtures having a fine granulation, in an alcoholic solution, with an organic compound of titanium or silicon, after which the organic solvent is removed, the remaining solid is dried at high temperature And ground and calcined at EN 111740 Bl of 3OO ... 6OO ° C, for 0.5 ... 4 h, when the titanium and / or silicon organic compounds are decomposed to the corresponding oxides. 2. Process according to claim 1, characterized in that, for obtaining modified metal oxides, the quantity of preliminary organic compounds of titanium and / or silicon is determined such that after calcining the metal oxides their specific surface is provided with a layer consisting of 1 to 5 titanium and / or silicon dioxide monolayers. Process according to claims 1 and 2, characterized in that, as preliminary organic compounds of titanium and / or silicon, compounds of the general formula Ti (0R) ₄ and Si (0R) _{4 are used} , wherein R is a organic radical. 4. Process according to claims 1 ... 3, characterized in that the dispersion layer of aluminum oxides, titanium, silicon, zeolite or mixtures thereof, which intensify the activity of the catalyst, is present in a concentration of 30. ..250, preferably 75 ... 180, or 90 ... 150 g / dm ³ volume of catalyst, vanadium calculated as V ₂ 0 ₅ is in a concentration of 0.1 ... 15 g / dm ³ volume of catalyst, and the metals in the platinum group, preferably platinum or palladium, are present in a concentration of 0.1 ... 7 g / dm ³ of catalyst volume. 5. Process according to claims 1 ... 4, characterized in that, as a body with a rigid structure, it uses a ceramic or monolithic support, preferably in the form of a monolith honeycomb, having a cell density of 5 to 100 cells / cm ^2. .